The recycling of different materials and the conversion of the energy contents of waste becomes ever more relevant and the target of large scale investments, private as well as public. The procedure used normally is to pre-sort those materials found in the municipal waste which may be recycled. What remains are materials that may be converted into energy through, for example, incineration.
The problem of waste accumulation grows steadily. It is expensive both in terms of collection costs and fees charged, and is particularly unacceptable in terms of the valuable resources that are wasted.
Incineration is a common method for retrieving energy from waste that can not be recycled. The energy is usually converted into heat in the form of hot water or steam that subsequently may replace “nobler” forms of energy.
Hierarchically, electricity in the form of alternating current is the most valuable form of energy, as it may, at low cost, be converted into practically any other form of energy.
The cost of converting the energy contents of waste into electric power through incineration is, however, very high, usually requiring large plants if conversion is to yield a good financial return. The high costs are the result particularly of very strict public regulations regarding the release of polluting emissions into the atmosphere.
Pyrolysis is known traditionally as the conventional retort dry distillation process utilising an external source of heat, sometimes combined with partial combustion of the contents in the retort. Examples of such processes are the production of wood-tar/charcoal and coke from wood and coals respectively. In modern plants, pyrolysis may be best known as what the petrochemical industry refers to as “cracking.”
Even after the invention of the magnetron in 1921, which used radio tubes to create the microwaves (MW), it took a quarter of a century before MW generators reached the market and came into practical use. In the post-WWII period, MW processes have found multiple applications within industries and in private households.
MW technology allows for the conversion of waste into electricity and other energies in smaller units than used in conventional incineration because of the minute quantities of air involved in the process. This results in considerably smaller volumes of gasses and vapours per unit of energy gained, as compared with incineration.
Pyrolysis is the equivalent of dry distillation and involves the decomposition of materials at a precisely controlled temperature, with or without precisely controlled quantities of oxygen, air or other additives for the enhancement of the process. This enables the extraction of valuable chemicals from the waste that would otherwise have been destroyed had the waste been burned.
The fractionated component materials that may be obtained through microwave pyrolysis (MWP) fall roughly into five categories.
1—Non-condensing volatile gas, which in many cases may be utilised directly as fuel in an IC engine/gas turbine to generate electricity after only simple cooling, rinsing and filtering.
2—Liquids, which may be utilised as fuel oils or, after fractional condensation, may represent raw materials of interest to chemical industries.
3—Carbon. This is the charred end product of organic waste which may be further processed into activated charcoal, be utilised as a reducing agent in metallurgical processes, or be used as a solid fuel whose flue gas emissions are exceptionally clean.
4—Metals, such as the metal cording from shredded automobile tires, will after MWP maintain their strength and resilience and may be added to reinforced concrete or plastics. Had the steel been exposed to ordinary incineration, it would have become oxidised and would thereby have lost its value both as an additive and as a raw material. Copper from electric cables and metals from electronic waste are other examples of recovered metal.
5—Ashes. Metals and minerals may in many instances be re-circulated by being separated from the ash fraction after MWP. Examples of such recycling are the recovery of chrome from tanning industry waste and of silver from exposed x-ray films.
A disadvantage of conventional dry distillation or pyrolysis is that, for the most part, the heat for the raw materials must come from outside the retort. This delays the heat transfer and leads to uneven decomposition temperatures inside the retort.
In MWP the heating takes place by volume inside the raw material or the waste itself. Thus, a considerably more efficient conversion may take place as compared to retort pyrolysis. The process temperature may be controlled to within narrow parameters. The state of the art is such that retort pyrolysis by batch as well as continuous MWP using linear conveyors are available technologies.
The known methods are, however, relatively inflexible if the raw material is of varying consistency and properties. Certain materials resist being heated as they will not absorb MW. These materials have to be heated indirectly, often by mixing shredded MW-inert material together with MW-absorbing material such as carbon, or other MW-reactive materials.
Decomposition of materials and MW absorbing waste through MWP has been known for many years, and is referred to in several patents, such as: U.S. Pat. No. 3,843,457 & U.S. Pat. No. 5,330,623.
Fractional condensation for the extraction of volatile and liquid components is referred to in U.S. Pat. No. 3,843,457.
Mixing of materials that do not absorb MW with granulated carbon or other MW absorbent materials is referred to in Norwegian patent application: NO-1995,2652.
Preheating of raw materials with infrared light (IR), prior to MWP is referred to in U.S. Pat. No. 5,084,141 and U.S. Pat. No. 4,647,443.
The use of inert gas in the pyrolysis chamber to prevent oxidisation of the raw materials is referred to in U.S. Pat. No. 5,084,141.
A straight conveyor for the transport of the raw materials for decomposition through a tunnel is likewise referred to in U.S. Pat. No. 5,084,141.
Full or partial use of volatile, flammable gas components from MWP for the preheating of raw materials or wastes is referred to in U.S. Pat. No. 5,084,141.
The generating of electricity through the use of volatile flammable gases used directly as motor fuel after filtering has been described in several sources and has been utilised in industrial processes.
MW technology has been, is and will be used in such industrial plants and processes as                The sterilising and/or destruction of infectious material as: prions, fungi spores, viruses, bacteria and other undesirable micro-organisms.        The sterilising and/or decomposition of liquid organic waste such as: sludge from sewage plants, and oil-polluted sludge from petrochemical drilling.        Recovering of metals, salts or minerals, such as chrome from tanning industry waste.        Recycling and recovering of waste components found in municipal wastes such as: energy, plastics and glass.        Decomposition or destruction of “problem wastes,” such as: PCB, dioxin, anti-fouling for ships hulls, paints, etc.        Recycling of chemicals and energy from plastics and rubber products, such as: automobile tires and electronic products.        Energy production from compressed agricultural waste, such as: straw        Hardening processes for organic adhesives, steels and different mineral compositions.        Welding operations for thermoplastics.        
It may appear after the above summary that MWP is a fully developed technology and that there no longer is room for improvement. That, however, is only how it seems at first glance. If one hoped to acquire full scale equipment for, for example, the treatment or decomposition of large quantities of tar impregnated railway sleepers or for the recycling of automobile tires on a large scale, one would discover that there is no such commercially operative equipment available to treat these problematic wastes, none with the capacity adapted to the magnitude of the problem.